Corrosion resistance of black oxide vs. zinc-iron phosphate?

A. This sounds like a benign environment where the very limited corrosion resistance of black oxide won't be a problem. But this Acme screw sounds like a moving part that requires lubricity in the finish, and I think that phosphatization offers substantially more than black oxide. I doubt that a black oxided ball screw will be a good idea. But black oxide may be what you get anyway, regardless of what you specify, if you're not there to ensure compliance.

A. Dear Bob,

Black Oxidising on ferrous metal, also called as blackodising, is a decorative finish and has very limited corrosion resistant properties. A suitable grade of Rust Preventive oil will increase the life. If a dry film as top coat is permissible, a thin film of clear PU, epoxy or other lacquer will give far better life.

Zinc-iron, zinc-calcium, manganese-iron phosphating with suitable anti-rust oil will give longer life. Manganese phosphating produces a dark grey finish. Alternatively, zinc plating with black passivation will serve the purpose.

A. Since phosphate coatings and oxide coatings are very low corrosion resistance at low temperature, you may go for zinc plating with olive green conversion coating. It gives better corrosion resistance (more than 100 hrs salt spray comparing less than an hour for phosphate or oxide). Moreover this coating give adequate lubrication property.

A. Hi Sunil. As you can see, we appended your inquiry to a thread which partially answers it. "Blackodizing" is a term used in India to describe the same process that is called "Black oxiding" here in the USA. It is not corrosion resistant. If you have a humid atmosphere, zinc plating might be a better choice, as it's the least expensive plating.

But in truth, mild steel components receive all kinds of metal platings and/or organic coatings, so it's hard to suggest the best one without a lot of information about your requirements. Good luck.

Regards,

A. Hi Kunal,

Black oxide coatings on their own offer very little in the line of corrosion resistance, they need a supplemental treatment to improve that aspect.

Common supplemental treatments vary from simply oiling them to applying coatings such as varnishes. The final use for the parts will determine which supplemental treatment you will require.

A. Hi cousin Kunal. As Brian implies, the ball is in your court as we await a description of your parts and their application -- all we can do is try to help with your specific situation.

But I raise a red flag when you say "normal weather conditions" -- outdoor exposure is not what black oxide is about. It can only be used outdoors in very limited situations like lovingly maintained firearms that are cleaned and oiled after each field trip. You may have the wrong finish for the application. Good luck.

Regards,

A. Hi Bharath. Sorry, I don't know and couldn't find anything on google easily -- but I suspect that ethylene glycol is harmless to phosphate coatings.

But the thing to remember is that commercial radiator coolant is not water plus ethylene glycol ... it is those two things PLUS about 10% corrosion inhibitors. Your tank will probably survive fine with commercial radiator coolant, but I would not bet on it surviving with just a 50-50 mix of water and ethylene glycol. Good luck.

Regards,